The present invention relates to an image reproduction system such as a printer operable as an output device of an analog facsimile transceiver or of a computer system, in which image information is exchanged in the form of analog signals as typified by frequency or amplitude modulated signals. More particularly, the present invention is concerned with a method of and apparatus for compensating images in such an image reproduction system which promote high quality image reproduction.
In communication through wire such as facsimile, major causes for the deterioration to reproduced images generally include group delay, attenuation distortion and signal-to-noise or S/N ratio. Of these causes, the group delay and attenuation distortion appear themselves in a reproduced image as ghost. Use has conventionally been made of a phase equalizer corresponding to 1.5-2 links against the group delay and an attenuation equalizer of 4-8 dB against the attenuation distortion. These means are substantially effective to eliminate the ghost.
The S/N ratio is lowered due mainly to the generation of random noise and this results in smear in a reproduced image. No effective measure is presently available against random noise which is economically applicable to facsimile transceivers and others which are to be produced on quantity basis. In conjunction with the art of satellite communication systems and precision measuring instruments, it has been known to pick up effective signals out of signals buried in noise, that is, signals of low S/N ratios. The S/N ratio may be improved as by employing autocorrelation or by repeatedly processing the same input signal to obtain its average. A drawback inherent in this kind of technique is the need for a disproportionate cost. Hence, it is unsuitable for facsimile and other apparatuses manufactured by quantity production though applicable to extensive ones such as satellite communication systems and precision measuring instruments.
The problem of random noise is particularly severe when the background density is too high to show a sufficient contrast relative to characters printed thereon, as is the case with newspapers for example. The random noise makes the code length longer during coding of an image signal for transmission and, accordingly, increases the time required for the transmission of the signal.
In conventional analog facsimile apparatuses, random noise has been compensated for by making an arrangement such that a reference voltage for the analog-to-digital conversion of a demodulated signal into a binary code is adjustable to a suitable value. This implement still involves a problem that random noise sharply grows to a noticeable degree in response to a fluctuation, particularly deterioration, in S/N ratio during transmission or reception of modulated signals.
Such a problem may be solved by the known MTF (modulation transfer function) compensation. Actually, it has been practiced in an analog facsimile system to carry out the MTF compensation at a transmitter station to compensate an image to be transmitted. However, the MTF compensation at the transmitter station tends to generate random noise against the intention and, rather, makes the reproduced image more illegible.